Pressure transducers have been employed in a myriad of applications. One such transducer is the capacitive manometer which provides very precise and accurate measurements of pressure of a gas, vapor or other fluid. Applications include precision control of vacuum based processes and semiconductor process control. Examples include semiconductor etch process and physical vapor deposition.
Capacitive manometers typically use (a) a flexible diaphragm forming or including an electrode structure and (b) a fixed electrode structure spaced from the diaphragm so as to establish capacitance there between. Variations in pressure on one side of the diaphragm relative to the pressure on the opposite side of the diaphragm causes the diaphragm to flex so that the capacitance between the electrode structure of the diaphragm and the fixed electrode structure varies as a function of this differential pressure. Usually, the gas or vapor on one side of the diaphragm is at the pressure being measured (Px), while the gas or vapor on the opposite side of the diaphragm is at a known reference pressure (Pr), the latter being at atmosphere or some fixed high or low (vacuum) pressure, so that the pressure on the measuring side of the diaphragm can be determined as a function of the capacitance measurement.
Many applications requiring extremely low pressures (high vacuum) have been and continue to be developed resulting in the need for capacitive manometers capable of measuring such low pressures. Increasing the sensitivity of capacitive manometers to provide very precise and accurate pressure measurements at low pressures, however, poses several design challenges. In order to measure extremely low pressures (high vacuum), a capacitive manometer typically requires a very narrow gap between the flexible diaphragm and the fixed electrode structure (the “electrode gap”) so that small changes in pressure can be detected.
A drawback to using a very narrow electrode gap is that smaller changes in the shape of the electrode gap unrelated to the measurement of differential pressure across the diaphragm are also detected. One of these detrimental changes to the electrode gap shape is a change in the shape of the diaphragm by process-related chemical reactions such as the diffusion of gas molecules or atoms into a surface of the diaphragm. Capacitance measurements are based on the well known equation for parallel plate capacitance C:C=ereoA/s,  (EQ. 1)
where C is the capacitance between two parallel plates,
eo is the permittivity of free space,
er is the relative permittivity of the material between the plates (for vacuum, er=1),
A is the common area between the plates, and
s is the spacing between the plates.
Based on this equation, one can derive the relationship that the fractional change in capacitance is equal to the negative of the fractional change in electrode gap spacing for each measuring electrode (ΔC/C=−ΔS/S).
It can be readily seen that it is critical to maintain good control over the electrode gap spacing in order to provide stable control over the capacitance of each measuring electrode. In a simple dual electrode design, these effects are balanced to a first order at zero differential pressure for a flat diaphragm and electrode structure (each having different real values of flatness and inclination deviation from true plane) for a given electrical measurement technique such as with any number of commonly used bridge designs (e.g., the Wheatstone bridge, etc.) and/or other electrical measuring methods. Since the sensor is configured to measure extremely low pressures (extremely small diaphragm deflections), just balancing the electrodes without making a stable electrode gap is not enough to reduce the uncertainty of the pressure measurement to adequately low levels in order to accomplish stable detection of the smallest pressures.
As the capacitive measurements are designed to detect changes in displacement between the fixed electrode structure and the diaphragm pressure resisting element, one source of error relates to any changes in the shape and position of the diaphragm (as it affects the electrode gap), which can produce changes in the sensor output that are unrelated to pressure.
FIG. 1 is a diagram showing a side and top view in views A and B, respectively, of a portion of a prior art capacitance manometer 100. The device includes a diaphragm 102 spaced apart from an electrode structure 104. The electrode structure 104 includes an inner electrode 106 and an outer electrode 108 separated by a gap 110. As shown in view B, the electrodes can have a circular configuration. When a pressure differential exists between the pressure on both sides of the diaphragm 102, the diaphragm is caused to deflect, as shown by alternate position 102′.
FIG. 2 is a diagram showing side and section views in views A and B, respectively, of a portion of another prior art capacitance manometer 200. The device is similar to the one shown in FIG. 1 and includes a housing 201 with a diaphragm 202 spaced apart from an electrode structure 204. The electrode structure 204 includes an inner electrode 206 and an outer electrode 208 separated by a gap 210. The housing includes an inlet 212 for admitting gas to the region adjacent the diaphragm 202. A baffle 214 is present to control entry of the gas to the region adjacent the diaphragm 202. As shown in view B, the baffle may be secured to the housing 201 by multiple tethers 218. The baffle 214 has a solid shape with no interior features. In operation, gas from the inlet 212 goes around the baffle 214 and reaches the outer edge of the diaphragm first 202. The gas then spreads toward the center of the diaphragm 202.
FIG. 3 is a diagram showing side and alternate section views, in A-C, respectively, of a portion of a further prior art capacitance manometer 300. The device is similar to the one shown in FIG. 2 and includes a housing 301 with a diaphragm 302 spaced apart from an electrode structure 304. The electrode structure 304 includes an inner electrode 306 and an outer electrode 308 separated by a gap 310. The housing 301 includes an inlet 312 for admitting gas to the region adjacent the diaphragm 302. A baffle 314 is present to control entry of the gas to the region adjacent the diaphragm 302. As shown in view B, the baffle includes multiple apertures 316 distributed in a uniform distribution over all or the majority of the baffle 314. View C shows an alternate configuration of the baffle, with a uniform radial distribution of apertures 316 over all or the majority of the baffle 314.
Reference is made to U.S. Pat. Nos. 7,757,563; 7,706,995; 7,624,643; 7,451,654; 7,389,697; 7,316,163; 7,284,439; 7,201,057; 7,155,803; 7,137,301; 7,000,479; 6,993,973, 6,909,975; 6,901,808, 6,735,845; 6,672,171; 6,568,274; 6,443,015, 6,105,436; 6,029,525; 5,965,821; 5,942,692; 5,932,332; 5,911,162; 5,808,206; 5,625,152; 5,271,277; 4,823,603; 4,785,669 and 4,499,773; and U.S. Patent Published Application Nos. 20090255342; 20070023140; 20060070447; 20060000289; 20050262946; 20040211262; 20040099061; all assigned to the assignee of the present disclosure; the entire content of all of which patents and patent publications are incorporated herein by reference.
While such prior art manometers may be suitable for their intended purpose, they can never the less be prone to transient measurement errors, particularly when used with reactive gases such as atomic fluorine and the like.